Microporous membranes are known for a long time (e.g., see Non-Patent Reference 1), and are widely used for filters for filtration, etc. Microporous membranes include ones produced from a starting material of cellulose ester (e.g., see Patent References 1 to 7), ones produced from a starting material of aliphatic polyamide (e.g., see, Patent References 8 to 14), ones produced from a starting material of polyfluorocarbon (e.g., see Patent References 15 to 18), ones from a starting material of polypropylene (e.g., see Patent Reference 19), etc. These microporous membranes are used for filtration and sterilization for washing water for electronic industry, water for medicines, water for production of medicines, water for foods, etc., and their applications and amounts to be used are enlarging these days, and in particular, microporous membranes having high reliability are specifically noted from the viewpoint of collecting particles. Above all, microporous membranes of a crystalline polymer are excellent in chemical resistance, and in particular, membranes produced from a starting material of polytetrafluoroethylene are excellent in heat resistance and chemical resistance, and the demand for them is increasing remarkably.
In general, the filtration capacity of a microporous membrane per the unit area is small (that is, the filtration life is short). Accordingly, in industrial use, a large number of filtration units must be combined in parallel with each other and used for increasing the membrane area, and from the viewpoint of reducing the cost in a process of filtration, it is desired to prolong the filtration life. From this viewpoint, heretofore, as a membrane effective for preventing flow rate reduction owing to pore blocking or the like, an asymmetric membrane has been developed, in which the pore size gradually decreases from the inlet side toward the outlet side of the liquid to be filtered through it (see Patent References 20 and 21). In addition, proposed are a polytetrafluoroethylene double-layered porous membrane that comprises a filtration layer having a small pore size and a support layer having a pore size larger than that of the filtration layer (see Patent Reference 22), and a membrane produced by applying a polytetrafluoroethylene emulsified dispersion onto a polytetrafluoroethylene sheet and stretching it (see Patent Reference 23).    Patent Reference 1: U.S. Pat. No. 1,421,341    Patent Reference 2: U.S. Pat. No. 3,133,132    Patent Reference 3: U.S. Pat. No. 2,944,017    Patent Reference 4: JP-B 43-15698    Patent Reference 5: JP-B 45-3313    Patent Reference 6: JP-B 48-39586    Patent Reference 7: JP-B 48-40050    Patent Reference 8: U.S. Pat. No. 2,783,894    Patent Reference 9: U.S. Pat. No. 3,408,315    Patent Reference 10: U.S. Pat. No. 4,340,479    Patent Reference 11: U.S. Pat. No. 4,340,480    Patent Reference 12: U.S. Pat. No. 4,450,126    Patent Reference 13: German Patent 3,138,525    Patent Reference 14: JP-A 58-37842    Patent Reference 15: U.S. Pat. No. 4,196,070    Patent Reference 16: U.S. Pat. No. 4,340,482    Patent Reference 17: JP-A 55-99934    Patent Reference 18: JP-A 58-91732    Patent Reference 19: West German OLS 3,003,400    Patent Reference 20: JP-B 55-6406    Patent Reference 21: JP-B 4-68966    Patent Reference 22: JP-A 4-351645    Patent Reference 23: JP-A 7-292144    Non-Patent Reference 1: R. Kesting's Synthetic Polymer Membrane (published by McGraw Hill)